How to fix «Element 'link', attribute 'href': The attribute 'href' is not allowed» / «The attribute 'src' is required but missing» after upgrading Magento to 2.2?

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In my case, the design theme contained the following declaration:

<link rel="stylesheet" type="text/css" href="https://cloud.typography.com/7742494/6841152/css/fonts.css" src_type="url"/>

I has changed it to:

<link src="https://cloud.typography.com/7742494/6841152/css/fonts.css" src_type="url"/>